The present invention relates to the blocking of an ophthalmic lens blank preparatory to a lens grinding operation and, in particular to a lens blocking machine which adheres a lens to a block insert by means of a low temperature alloy.
The manufacture of ophthalmic lenses involves the placement of a finished surface on one of the major faces of a lens blank, and the grinding of a prescription surface on the other major face thereof. Usually the finished surface is convex and is formed during the fabrication of the lens blank. The prescription surface is usually concave and ground in the field. The prescription surface is ground about an axis, termed a cylinder axis, disposed in a particular orientation relative to a reference axis of the lens blank.
The radius of the prescription surface is set at a required length to correct for the amount of myopia or hyperopia. In certain instances, such as if astigmatism is to be corrected, the cylinder axis is angularly offset relative to the reference axis of the lens to create a toric lens configuration.
Grinding is carried out in a grinding machine which comprises a lens holder and a grinding tool. Typically, the grinding step is carried out by passing the grinding tool across a face of a stationarily-held lens. The lens is temporarily affixed to a block which is installed in a fixed location in the holder.
It is necessary that the optic center of the finished lens be properly oriented relative to the patient's pupil position. A relatively inexpensive way of achieving this has been to form a prism in the lens. This is achieved by adjusting the position of the lens relative to the grinding tool to define a proper prism angle and axis. Since the block to which the lens is affixed is installed in a fixed location in the holder, it is necessary to set the proper prism angle axis when affixing the lens to the block. The lens is typically affixed to the block by means of a lens blocking mechanism comprising a wedge-shaped dial arrangement which positions the lens at a variable inclination relative to the block. By rotating the dial arrangement and clamping the lens to the dial arrangement, the lens can be selectively inclined relative to the block. Then, a molten metal alloy is fed from a pressurized tank into a cavity formed by the lens, the dial and the block. Upon hardening, the metal adheres to the lens and block to form a releasable bond therebetween.
It would be desirable to improve and simplify the operation and performance of the dials in order to enable the prism angle to be set more rapidly and accurately.
It would also be desirable to improve the safety of operation of the blocking machine. In that regard, the lens is secured to a seat of the dial mechanism by being pushed firmly thereagainst by a pneumatic clamp. It may occur, however, that the operator's finger may accidentally become pinched between the clamp and the lens whereby an injury may occur. Another problem involves the possibility that upon a release of the clamp, molten alloy under residual pressure in the alloy tank may spurt out past the unclamped lens.
Another shortcoming relating to conventional lens blocking equipment involves the blocking of so-called progressive lenses which contain a recessed bifocal section. The presence of such a recessed section makes it difficult to create a proper seal between the lens and the lens seating surface. In the past, this problem has been dealt with by cramming putty into the gap created by the recessed section. However, this procedure is difficult and messy and requires that a supply of putty be kept on hand.